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Field  Columbian  Museum 

Publication    ioi. 
Geological  Series.  Vol.   Ill,   No.    i 


THE    RODEO    METEORITE 


BY 


Oliver  Cummings  Farrington,   Ph.  D. 
Curator,  Department  of  Geology. 


Chicago,    U.  S.   A. 
March   i,  1905 


Field  Columbian  Museum 

Publication   ioi. 

Geological  Series.  Vol.  Ill,  No.   i. 


THE    RODEO    METEORITE 


BY 


Oliver  Cummings  Farrington,   Ph.D. 
Curator,  Department  of  Geology 


Chicago,  U.  S.  A. 

March  I,  1905 


THE    RODEO   METEORITE. 

BY  OLIVER  CUMMINGS  FARRINGTON. 

This  meteorite  is  an  iron  mass  found  about  1852  by  a  goat-herder 
in  an  arroya  north  of  the  Nazas  River,  12  kilometers  northwest  of 
the  hamlet  of  Rodeo,  State  of  Durango,  Mexico.  The  location  is 
approximately  250  20'  N.  lat.  and  1040  40'  W.  long.  Upon  discovery 
of  the  iron  it  was  made  to  do  duty  as  an  anvil  at  a  forge  for  many 
years.  As  received  at  the  Museum,  evidence  of  its  industrial  use 
was  to  be  seen  in  its  having  been  beaten  flat  and  smooth  on  one  side. 
The  surface  so  treated  is  apparent  in  Plate  II.,  by  its  smoothness  and 
turned-over  edges.  The  meteorite  as  a  whole  is  irregular  in  form 
and  without  marked  orientation.  Its  extreme  dimensions  are  12  x 
9x8  inches  (30  x  23  x  20  cm.).  Its  weight  when  received  was  97 
pounds  (44.1  kgs.).  An  attempt  had  evidently  been  made  at  some 
time  to  cut  off  a  portion  of  the  mass  with  a  cold-chisel,  thus  producing 
the  incision  shown  in  Plate  I.  Above  this  a  small  surface  appears 
that  was  filed  smooth  for  etching.  In  other  respects  the  surface  of 
the  meteorite  has  the  natural  contours.  The  surface  in  general, 
though  irregular,  is  everywhere  rounded,  showing  no  angular  or 
sharp  edges.  There  are  many  partially  defined  pittings  of  various 
depths  and  diameters,  the  largest  of  these  having  an  elliptical  out- 
line and  being  4  inches  (10  cm.)  in  length,  3  inches  (8  cm.)  in  width, 
and  about  1%  inches  (4  cm.)  in  depth.  The  position  and  character 
of  this  pit  are  shown  in  Plate  I.  In  color  the  surface  of  the  meteorite 
is  darkened  by  exposure,  but  it  has  nowhere  rusted  deeply,  and  in 
several  places  the  nickel- white  color  of  the  metal  is  visible.  In  such 
places  Widmanstatten  figures  often  can  be  seen  also.  On  any  pol- 
ished surface  of  the  meteorite,  too,  the  figures  appear  nearly  as 
plainly  as  after  etching. 

Several  complete  sections  of  the  meteorite  were  made  in  order  to 
determine  its  interior  structure.  All  show  on  etching  well-defined 
figures  octahedral  in  character.  A  photograph  of  one  of  these  etched 
sections  is  shown  in  Plate  III.  The  bands  (Balken)  are  more  numer- 
ous than  the  meshes  (Felder),  yet  the  latter  occupy  a  considerable 
amount  of  the  total  area.  Through  a  belt  about  two  inches  (5  cm.) 
in  width  running  across  the  middle  of  most  of  the  sections  a  minutely 
dotted  appearance  is  presented  resembling  that  described  by  Brezina 


2  Field  Columbian  Museum — Geology,  Vol.   III. 

as  characterizing  Charcas*  and  referred  by  him  tentatively  to 
minute  inclusions  of  troilite.  An  examination  of  the  dots  in  Rodeo 
with  the  lens  shows  them  to  be  minute,  shallow,  saucer-shaped 
pits.  They  are  scattered  irregularly  along  the  bands  of  kamacite, 
and  are  to  be  seen  in  some  of  the  swathing  kamacite,  but  never 
in  the  plessite.  The  tendency  of  the  iron  to  rust  at  these  points 
is  greater  also  than  at  others.  They  appear  therefore,  to  mark 
the  occurrence  of  some  more  soluble  ingredient  in  the  kamacite. 
This  is  probably  not  troilite,  but  may  be  an  iron  containing  less 
nickel  than  the  kamacite.  The  lamellae  of  the  meteorite  may  be 
grouped  into  two  classes;  one  about  i  mm.  in  width,  swollen,  and 
with  wavy  outlines,  and  the  other  about  half  as  wide,  and  with  more 
nearly  rectilinear  outlines.  As  a  rule,  these  two  kinds  of  lamellae 
have  a  different  orientation  as  compared  with  each  other.  The 
kamacite  is  granular,  much  lighter  in  color  than  the  plessite.  A  con- 
siderable quantity  of  swathing  kamacite  is  present.  While  in  general 
it  follows  the  outline  of  the  inclusions  and  forms  a  narrow  border  to 
them,  at  times  its  outer  border  is  quite  independent  of  the  shape  of 
the  inclusions  and  it  covers  relatively  broad  areas.  The  taenite  is 
well  developed,  silver-white  in  color,  and  displays  the  structure  of  a 
section  brilliantly  on  holding  one  at  an  angle  to  the  light.  The  ples- 
site is  not  depressed  by  etching  as  is  the  kamacite.  At  times  it  occu- 
pies the  meshes  alone,  while  again  the  meshes  may  display  elaborate 
combs  resulting  from  skeleton  growths  of  taenite.  Scattered  irregu- 
larly through  the  sections  and  forming  an  important  feature  in  the 
structure  of  the  meteorite,  occur  numerous  inclusions  of  schreibersite. 
The  form  of  these  inclusions,  especially  those  of  large  size,  is  in  gen- 
eral elongated,  and  rectangular  or  spindle-shaped.  Some  of  the 
smaller  inclusions,  however,  are  star-shaped,  while  others  have  no 
well-defined  form.  The  largest  inclusion  noted  (shown  in  the  upper 
right-hand  corner  of  Plate  III.)  has  a  length  of  one  and  a  half  inches 
(4  cm.)  and  a  width  of  one-fourth  of  an  inch  (.5  cm.).  The  schreiber- 
site is  tin-white  in  color,  brittle,  and  magnetic,  and  affords  the  usual 
blow-pipe  and  chemical  tests  for  that  mineral.  The  inclusions  are 
always  bordered  by  a  band  of  swathing  kamacite  about  1.5  mm.  in 
width.  The  inclusions,  while  having  no  apparent  regularity  of 
arrangement  among  themselves,  are  usually  disposed,  especially  the 
elongated  ones,  parallel  to  the  Widmanstatten  figures,  or  in  other 
words,  the  octahedral  structure  of  the  meteorite.  This  can  well  be 
discerned  by  a  study  of  Plate  III.  Another  inclusion  of  an  interesting 
character  found  in  one  of  the  sections  was  a  nodule  about  one  centi- 
*Wiener  Sammlung,  1895,  p.  275. 


Mar.,  1905.      The  Rodeo  Meteorite — Farrington.  3 

meter  in  diameter,  of  a  black,  amorphous,  friable  substance  resem- 
bling graphite.  The  form  of  the  nodule  in  the  direction  of  the  section 
is  nearly  circular,  but  in  the  third  dimension  its  extent  is  unknown, 
as  it  penetrates  into  the  main  body  of  the  meteorite,  which  has  not 
yet  been  cut.  No  band  of  swathing  kamacite  surrounds  the  nodule, 
it  being  set  bodily  into  the  mass  of  the  iron.  In  appearance  and 
physical  properties  the  substance  of  the  nodule  resembles  graphite 
fully,  but  it  is  magnetic  and  fuses  in  the  reducing  flame  at  about  4. 
Mixed  with  potassium  nitrate  it  deflagrates  readily,  but  throws  out 
incandescent  sparks  in  addition  to  the  flaming  usual  to  graphite. 
Potassium  carbonate  results  from  the  reaction.  Oxidation  with  sul- 
phuric and  chromic  acids  according  to  the  French  method*  affords  an 
appreciable  quantity  of  CO 2.  On  heating  in  oxygen  the  substance 
glows  and  becomes  of  a  red-brown  color.  It  was  found  to  be  little,  if 
any,  attacked  by  the  ordinary  acids.  After  a  long  treatment  with  aqua 
regia,  however,  and  addition  of  ammonia  to  the  solution,  a  slight  pre- 
cipitate of  iron  hydroxide  was  obtained.  When  powdered  and  added  to 
a  copper  sulphate  solution,  copper  was  reduced  by  the  substance.  Its 
specific  gravity  (obtained  by  Thoulet's  solution)  was  2.38.  -On  account 
of  the  above  properties  it  would  appear  that  the  substance  is  chiefly 
graphite,  but  contains  in  addition  some  form  of  iron,  probably  a 
carbide,  intimately  mixed  with  it.  Such  a  mixture  should  exhibit 
the  properties  of  magnetism,  reduction  of  copper  from  copper  sul- 
phate and  insolubility  in  acids,  which  are  possessed  by  this  substance. 
Such  properties  seem  not  to  have  been  possessed  by  graphite  which 
has  been  described  from  other  meteorites.  It  is  common  for  meteoric 
graphite  to  be  accompanied  by  troilite,  as  has  been  noted  by  Smithf 
and  other  authors.  The  Rodeo  graphite,  however,  seems  to  contain  no 
troilite.  A  specimen  labelled  graphite  in  the  Museum  collection  of  what 
is  probably  a  portion  of  a  "salamander"  from  the  Isabella  Furnace, 
Etna,  Pennsylvania,  was  found  to  exhibit  properties  much  like  those 
of  the  Rodeo  graphite  except  that  it  is  heavier,  the  specific  gravity 
being  5.56,  and  the  structure  is  foliated  rather  than  compact. 
Since  a  "salamander"  originates  by  the  accumulation  of  graphite 
and  graphitic  substances  in  smelting  operations,  it  seems  reasonable 
to  suppose  that  the  Rodeo  nodule  is  a  similar  segregation  of  graphite 
originally  more  or  less  disseminated  in  the  iron. 

An  analysis  of  the  meteorite  was  made  by  Mr.  H.  W.  Nichols, 
of  the  Department  of  Geology  of  the  Museum.  Material  for  analysis 
was  secured  by  drilling  a  half-inch  hole  to  a  depth  of  seven-eighths 

*  Blair,  The  Chemical  Analysis  of  Iron,  third  edition,  p.  136. 

t  Am.  Jour.  Sci.  3,  2,  p.  304,  433. 


4  Field  Columbian  Museum — Geology,  Vol.   III. 

of  an  inch,  and  rejecting  the  drillings  from  the  crust  portion.  For 
the  determination  of  iron,  nickel,  and  cobalt,  a  portion  of  1.3733 
grams  was  dissolved  in  strong  hydrochloric  acid.  Solution  took 
place  rapidly  and  completely,  only  a  few  unweighable  black  flecks 
being  left  after  oxidation  with  nitric  acid,  evaporation  to  dryness 
and  addition  of  water.  Iron  was  precipitated  three  times  by  treat- 
ment with  ammonia  and  ammonium  chloride  as  directed  by  Frese- 
nius,  except  that  a  large  excess  of  the  reagents  was  used.  After 
solution  with  sulphuric  acid  and  reduction  with  hydrogen  sulphide, 
the  determination  was  made  by  the  usual  titration  with  potassium 
permanganate.  Copper  was  precipitated  by  hydrogen  sulphide  from 
the  filtrate  from  the  iron  precipitate  and  then  determined  electro- 
lytically.  Nickel  and  cobalt  were  separated  in  acetic  acid  solution 
as  sulphides  and  separated  by  potassium  nitrite.  Nickel  was  then 
determined  electrolytically,  but  cobalt  as  sulphate,  the  electro- 
lytic determination  of  this  element  having  proven  at  times  unreliable. 
Manganese  was  tested  for  in  a  portion  of  2.8248  grams  dissolved  in 
nitric  acid  and  oxidized  with  potassium  chlorate  according  to  Ford's 
method.  No  precipitate  was  obtained.  The  treatment  with  nitric 
acid  showed  the  meteorite  to  be  passive  until  water  was  added. 
Sulphur  and  phosphorus  were  determined  in  a  portion  of  4.8321 
grams  dissolved  in  fuming  nitric  acid  by  the  slow  addition  of  hydro- 
chloric acid.  From  this  sulphur  was  precipitated  as  directed  by 
Blair  when  iron  is  present,  purified  by  fusion  with  sodium  carbonate, 
and  weighed  as  barium  sulphate.  Phosphorus  was  determined  by 
the  acetate  method  and  weighed  as  magnesium  pyrophosphate. 
Carbon  was  determined  in  a  portion  of  2.5678  grams  by  oxidation 
with  chromic  and  sulphuric  acids  and  weighed  as  carbon  dioxide. 
During  the  treatment  the  odor  of  hydrocarbons  was  observed,  similar 
to  that  obtained  in  the  solution  of  pig  iron.  This  indicated  that 
some  of  the  carbon  was  present  in  a  combined  form,  while  an  insoluble 
residue  showed  that  some  existed  as  graphite.  The  analysis  gave  the 
following  results: 

Fe 89.84 

Ni 8.79 

Co 0.28 

Cu 0.07 

P 0.80 

S 0.02 

C 0.09 

99.89 
The  composition  of  the  meteorite  is  thus  seen  to  be  that  usual  to 
medium  octahedrites,  with  a  high  percentage  of  phosphorus.     From 


Mar.,  1905.      The  Rodeo  Meteorite — Farrington.  5 

the  large  amount  of  schreibersite  visible  in  the  sections,  such  a  con- 
tent of  phosphorus  would  be  expected. 

Including  Rodeo,  the  meteorites  now  recognized  from  the  State 
of  Durango  are,  with  dates  of  their  fall  or  find,  as  follows: 

Avilez,  Spherulitic  chondrite,  Cc 1856 

Bella  Roca,  Fine  octahedrite,  Of 1888 

Cacaria,  Hammond  octahedrite,  Oh 1867 

Rodeo,  Medium  octahedrite,  Om 1852 

Rancho  de  la  Pila,  Medium  octahedrite,  Om 1804 

San  Francisco  del  Mezquital,  Siratik  ataxite.  Ds 1868 

The  localities  of  these  have  been  determined  as  accurately  as 
possible  by  the  writer  from  various  published  accounts,  and  are 
represented  as  determined  on  the  accompanying  map  (Plate  IV.). 
Of  these  meteorites  only  one,  Avilez,  is  a  stone;  the  others  are  all 
irons.  Of  the  irons,  Cacaria  and  San  Francisco  del  Mezquital  are 
sufficiently  distinguished  by  their  structure,  Cacaria  being  a  Ham- 
mond octahedrite  and  San  Francisco  del  Mezquital  an  ataxite. 
Rodeo  and  Rancho  de  la  Pila  are  both  medium  octahedrites,  but  the 
localities  from  which  they  come  are  about  seventy  miles  apart. 
Hence  only  Bella  Roca  needs  to  be  compared  with  Rodeo  in  order  to 
determine  whether  it  belongs  to  the  same  fall.  The  localities  of 
Rodeo  and  Santiago  Papasquiaro,  near  which  Bella  Roca  is  said  to 
have  been  found,  are  in  a  direct  line  about  forty  miles  apart.  This 
is  much  farther  than  parts  of  a  single  meteor  could  have  been  natu- 
rally distributed  according  to  our  present  knowledge.  That  they 
might  have  been  separated  by  human  agency  is  possible,  but  not 
probable,  since  the  country  between  these  localities  is  thinly  settled 
and  difficult  to  travel  over.  From  the  appearance  of  the  surface  of 
the  Bella  Roca  meteorite  Brezina  concluded*  that  it  could  have 
lain  exposed  but  a  little  while.  The  Rodeo  meteorite,  however, 
is  known  as  far  back  as  1852.  Brezina  also  describes  Bella  Roca 
as  a  highly  oriented  individual  showing  an  almost  complete  fusion 
crust.  Neither  of  these  observations  would  apply  to  the  Rodeo 
iron.  From  the  point  of  view  of  structure  the  two  irons  do  not 
differ  essentially.  The  appearance  of  the  lamellae  is  indeed  quite 
similar,  with  the  exception  that  Rodeo  has  a  preponderance  of  the 
wider  lamellae,  sufficient,  in  the  author's  opinion,  to  warrant  class- 
ing it  as  a  medium  octahedrite.  Bella  Roca  is  classed  as  a  fine 
octahedrite  by  Brezina.  There  is  also  a  similarity  between  the 
two  irons  in  the  fact  that  the  schreibersite  inclusions  follow  the 
octahedral  lamellae,  in  their  orientation.     The  schreibersite  in  Rodeo, 

*  Wiener  Sammlung,  1895,  p.  271. 


6  Field  Columbian  Museum — Geology,  Vol.   III. 

however,  is  considerably  stouter  in  habit  than  that  in  Bella  Roca. 
The  chief  point  of  difference  between  the  two  irons,  however,  and 
one  which  in  the  writer's  view  seems  alone  to  warrant  their  separa- 
tion, is  that  in  Rodeo  there  is  an  entire  lack  of  the  inclusions  of 
troilite  which  form  so  striking  and  important  a  feature  of  the  compo- 
sition of  Bella  Roca.  Although  eight  full-sized  sections  have  been 
made  of  Rodeo,  no  troilite  has  as  yet  been  observed  in  it.  In  Bella 
Roca,  however,  as  is  well  known,  troilite  is  an  abundant  and  charac- 
teristic constituent.  The  chemical  analyses  of  the  meteorites  do  not 
show  important  differences,  but  this  would  not  be  expected  as  between 
medium  and  fine  octahedrites.  Still  the  analyses  show  a  relative 
absence  of  sulphur  and  hence  of  troilite,  and  abundance  of  phos- 
phorus and  hence  of  schreibersite  in  Rodeo,  while  the  opposite  condi- 
tion holds  in  Bella  Roca.  The  analyses  compare  as  follows,  that  of 
Bella  Roca  being  by  Whitfield:* 

Fe         Ni       Co        Cu         P  S  C 

Bella  Roca.  .    91.48     7.92     0.22 0.21     0.21     0.06  =  100.10 

Rodeo 89.84     8.79     0.28     0.07     0.80     0.02     0.09  =    99.89 

In  view,  therefore,  of  the  distance  between  the  localities  and  the 
difference  in  structure  and  composition,  there  seems  to  be  sufficient 
reason  for  regarding  Rodeo  as  a  distinct  fall. 

*Am.  Jour.  Sci.  3,  37,  p.  439. 


FIELD   COLUMBIAN    MUSEUM. 


GEOLOGY,    VOL.    Ill,    PLATE    I. 


The  Rodeo  Meteorite,  x  y2.  The  incision  and  smoothed  surface  in  the  upper  right- 
hand  corner  are  of  artificial  origin.  The  remainder  of  the  surface  presents  the  natural 
appearance. 


UNIVERSITY  OP  ILL! 


FIELD   COLUMBIAN    MUSEUM. 


GEOLOGY,   VOL.   Ill,    PLATE    II. 


The  Rodeo  Meteorite,    x  l/2.    The  smoothed  surface  at  the  left  is 
due  to  the  use  of  the  meteorite  as  an  anvil. 


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